Abstract Alzheimer's disease (AD) is a progressive brain disease that severely affects memory, reasoning, and behaviors. Currently, there is no cure available for AD, making it one of the most unmet medical challenges in our society. In the AD, the formation and toxicity of ?-amyloid plaque and tau-positive neurofibrillary tangles are central to the AD pathology. Especially, both ?-amyloid plaque and tau positive neurofibrillary tangles propagate from entorhinal cortex (EC) to hippocampal regions, significantly spreading the toxicity and exacerbate the disease pathology. Recently, exosome-mediated secretion and intercellular communication has been implicated in disease initiation and propagation in AD. Exosomes (size 40-150 nm), a major type of secreted extracellular vesicles (EVs), are derived from intraluminal vesicles (ILV) that are budded inwardly from the early endosomal compartment, and are released from multiple vesicular bodies (MVBs), during endosome maturation. However, very little is currently known about whether and how exosome dynamics (ILV trafficking, exosome secretion and uptake) is altered by the expression of the mutant APP in CNS cells in the rodent (APPNL-F/NL-F mice) and human (fAD iPSC-derived neurons) AD models. Especially, the dynamic changes of the secretion and migration of cell-type specifically secreted exosomes in vivo in APPNL-F/NL-F mice and their potential toxicity to synapses has not been investigated. We have previously published that both neurons and glia secrete exosomes. We have recently generated a knock-in CD63-GFPf/f mouse line from which the copGFP (a variant of GFP)-fused CD63 (membrane marker of ILV/exosomes) can be induced in a Cre-dependent manner, allowing cell-type specific labeling and tracing of exosomes in situ in different (patho)physiological conditions. By employing this new tool, we further found that exosomes are abundantly present in vivo and secreted exosomes can migrate extensive distance from the initial secreted site. Based on previous studies and our preliminary results, we propose to investigate the following aims: 1) Determine the effect of mutant APP on exosome dynamics in CNS cells in mouse and human AD models; 2) Investigate cell-type specific exosome-mediated A? plaque propagation and toxicity mechanisms in the AD model. We have generated a large amount of preliminary data to support our rationales and to demonstrate feasibility for proposed aims. We will employ mouse genetics, virus injections, imaging, and biochemical approaches to investigate these two aims. Outcomes from this project will provide much needed new insights about exosome secretion/uptake in various CNS cells in APPNL-F/NL-F mice. Importantly, our in vivo investigation of cell-type specifically secreted exosomes and their association to the A? pathology in AD models, -by employing our newly generated exosome reporter (CD63-GFP cKI) mice, will be a significant step forward in understanding the in vivo roles of cell-type specific exosomes in AD pathogenesis, which will ultimately lead to new therapeutic opportunities.